The invention relates to an immersion heater having a housing which is inserted into liquid to be heated.
Immersion heaters for heating liquids, in particular water, have been known for a long time. The problem has existed to provide in every case a sufficient operating safety for such immersion heaters since the danger of fire with such apparatus is particularly great. Indeed, for an immersion heater the heating element can come directly in contact with materials such as a tablecloth, newspaper, etc. which are readily flammable. A protective screen for the heating element contained in the immersion heater, as has been readily employed in other kitchen appliances, cannot be practically realized for an immersion heater and could not normally be realized, at least up to now.
In conjunction with the protection of the heating resistor against overheating, the German Utility Model 1,962,119 has already suggested using a component known as a PTC thermistor as the heating resistor. A PTC thermistor is also referenced a ceramic PTC resistor. The PTC thermistor or resistor has a rapid resistance rise with a rising temperature within a relatively narrow temperature range. The location of the temperature range depends upon the material and depends upon the respective Curie-temperature of the material used for the PTC thermistor. The material per se is a ceramic material on the basis of barium titanate, whereby the position of the Curie temperature is determined, in particular, by the choice of a respective doping.
For the immersion heater with a PTC thermistor according to the above-captioned German utility model, however, no such useful technical disclosures are made which result in a functional device. To the contrary, disclosures, for example, for selection of the position of the temperature range for the resistance rise above mentioned are made which led one skilled in the art away from the inventive resolution described below.